feat: Add Z Anti-Aliasing (ZAA) contouring support

Port Z Anti-Aliasing from BambuStudio-ZAA (https://github.com/adob/BambuStudio-ZAA)
to OrcaSlicer. ZAA eliminates stair-stepping on curved and sloped top surfaces
by raycasting each extrusion point against the original 3D mesh and micro-adjusting
Z height to follow the actual surface geometry.

Key changes:
- Add ContourZ.cpp raycasting algorithm (~330 lines)
- Extend geometry with 3D support (Point3, Line3, Polyline3, MultiPoint3)
- Template arc fitting for 2D/3D compatibility
- Change ExtrusionPath::polyline from Polyline to Polyline3
- Add 5 ZAA config options (zaa_enabled, zaa_min_z, etc.)
- Add posContouring pipeline step in PrintObject
- Update GCode writer for 3D coordinate output
- Add ZAA settings UI in Print Settings > Quality
- Add docs/ZAA.md with usage and implementation details

ZAA is opt-in and disabled by default. When disabled, the slicing pipeline
is unchanged.

src/libslic3r/ExtrusionEntityCollection.hpp

@@ -118,8 +118,8 @@ public:
     ExtrusionEntityCollection chained_path_from(const Point &start_near, ExtrusionRole role = erMixed) const 
     	{ return this->no_sort ? *this : chained_path_from(this->entities, start_near, role); }
     void reverse() override;
-    const Point& first_point() const override { return this->entities.front()->first_point(); }
-    const Point& last_point() const override { return this->entities.back()->last_point(); }
+    Point first_point() const override { return this->entities.front()->first_point(); }
+    Point last_point() const override { return this->entities.back()->last_point(); }
     // Produce a list of 2D polygons covered by the extruded paths, offsetted by the extrusion width.
     // Increase the offset by scaled_epsilon to achieve an overlap, so a union will produce no gaps.
     void polygons_covered_by_width(Polygons &out, const float scaled_epsilon) const override;